Fluid sampling system

ABSTRACT

A fluid sampling system has a fluid enclosing element, such as drift cell (60), enclosing a volume of a first fluid. A body of a second fluid, for example in an inlet chamber (62), communicates with the body of first fluid via a small orifice (74). A series of negative pressure pulses is applied to the first fluid by an electromechanical transducer (92), each negative pulse causing a sample of the second fluid to be drawn in through the orifice (74). The sample is then entrained into the air flow of a closed loop circulatory system, and can be detected or measured by any appropriate equipment such as an ion mobility spectrometer.

TECHNICAL FIELD

The present invention relates to a fluid sampling system for effectingthe transfer of samples of a second fluid from a body of that fluid intoa system incorporating an enclosed volume of a first fluid. The firstfluid may or may not circulate within the system.

DISCLOSURE OF THE INVENTION

Such fluid sampling systems are commonly employed in analyticalinstruments e.g. chromatographs and mass spectrometers where it isneeded to transfer for analysis a small sample of gas, vapour or liquidof interest.

Such fluid sampling systems may also be employed, for example, inatmospheric monitoring or analysis equipment where a sample or samplesof an ambient atmosphere are to be transferred into a closed loopcirculatory system within the equipment and examined for the presence ofcomponents of interest.

Atmospheric monitoring equipment of this general type is described in UKPatent No. 2052750 in which an external atmosphere is sampled by drawinga flow of the external atmosphere over a membrane through which a sampleof the atmosphere permeates into a closed loop circulatory system and isentrained in the closed loop gas flow, and conveyed to means fordetecting and/or identifying vapours or gases of interest in theentrained sample.

Membrane inlet systems, such as employed in the equipment described inUK Patent No. 2052750, suffer a number of significant disadvantages. Forexample the membrane employed in such systems has a slow response tosampling commands, tends to retain sample from one sampling to the next,and often requires local heating to optimise sample permeation throughit. Most inconveniently the transmission characteristics of the membranecannot be varied, for example to permit variation of instrumentalsensitivity or dynamic measurement range.

It is an object of the present invention to provide a novel fluidsampling system in which the disadvantages of such prior art samplingsystems are largely overcome or at least mitigated.

According to an aspect of the present invention there is provided afluid sampling system comprising fluid enclosing means arranged toenclose a volume of a first fluid into which a sample of a second fluidis to be introduced, the fluid enclosing means having an orifice viawhich the second fluid may be drawn into the fluid enclosing means, andpressure pulse means arranged to apply a negative pressure pulse to thefirst fluid whereby a sample of the second fluid is drawn into the fluidenclosing means through the orifice. To put it in other words, theinvention provides a sample inlet system of the type referred to inwhich the body of the fluid to be sampled is separated from the enclosedfluid volume into which a sample of the body of fluid is to betransferred, by means including an orifice, and in which means areprovided for applying a pressure pulse to the enclosed fluid volumewhereby a sample of the fluid body is drawn into the enclosed fluidvolume through the orifice. Continuous transfer may be effected byapplying a repetitive pressure pulsing to the system incorporating theenclosed fluid volume.

Although repetitive pulsing will cause fluid flow through the orificefrom the body of the fluid into the enclosed fluid volume and viceversa, sample material incoming to the enclosed fluid volume will beimpelled into the enclosed fluid volume or, in the case of there being acirculatory flow within the system incorporating the enclosed fluidvolume, will be entrained in the flow and conveyed away from theorifice, resulting in each case in a net flow of sample fluid into theenclosed fluid volume.

The rate at which sample material is transferred through the orifice maybe controlled by variation of the amplitude, the repetition rate, or theduration of, the pressure pulses, or by a combination of two or more ofthose parameters.

The rate of transfer of sample material through the orifice may becontrolled automatically by controlling one or more of the parameters ofthe pressure pulses applied to the system incorporating the enclosedfluid volume in response to a measured value of the transferred fluidsample or of a component of interest in the transferred fluid sample.

The pressure pulses applied to the system incorporating the enclosedfluid volume may be generated by means of an electromechanicaltransducer in which an applied electrical signal generates a mechanicaldisplacement capable of producing pressure pulsing of the system. Byvarying the characteristics of the electrical drive signal to thetransducer, the parameters of the pressure pulses may be similarlyvaried.

The electromechanical transducer may be such as to produce pressurevariations in the system by positional variation of a diaphragm inresponse to the electrical signal applied to means displacing thediaphragm.

Such a transducer may be mounted in the system incorporating theenclosed fluid volume with the diaphragm in direct contact with thefluid in the system thereby enabling pressure pulses to be applieddirectly to the fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be carried into position in a number of ways and twospecific embodiments will now be divided, by way of example, withreference to the drawings, in which:

FIG. 1 is a diagrammatic representation of a fluid sampling system inaccordance with a first embodiment of the invention in association withan electron capture detector;

FIG. 2 is a plot of inlet flow through the sampling system of FIG. 1;

FIG. 3 is a plot of detector current with an electron-capturing samplematerial in the region adjacent the exterior of the sample inlet;

FIG. 4 is a schematic representation of an ion mobility spectrometerusing a fluid sampling system in accordance with a second embodiment ofthe invention; and

FIG. 5 is a diagrammatic representation of the electromechanicaltransducer system employed in the spectrometer of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an electron capture detector indicated generally at10 is mounted through one end wall 12 of an enclosure 14, and a smallloudspeaker 16 forms part of the exterior of the other end wall 18.

The rim 20 of the loudspeaker 16 is sealed to the exterior of the endwall 18 and the volume enclosed by the loudspeaker cone 21 communicateswith the interior of the enclosure 14 through apertures 22,22' in theend wall 18.

The electron capture detector 10 comprises a tubularelectrically-conducting outer body 24, and an inner electrode 26 mountedin the outer body 24 by means of an electrically insulating mount 28.The opposite end of the body 24 is closed by a plug 30 in which there isa pin-hole aperture 32 which communicates with the interior of the body24. The ionizing source for the detector is a cylindrical ten-millicurieNickel-63 foil sleeve 34 surrounding the open end of the inner electrode26 which is tubular and connected via a flexible electrically insulatingcoupling 36 to an inlet pipe 38 through which carrier gas for thedetector 10 is introduced. Carrier gas from the detector exits the body24 through apertures 40 and leaves the enclosure 14 through a pipe 42also mounted in the end wall 12.

Electrical connection is made to the inner electrode 26 by way of a lead44 which passes through the end wall 12 in the insulating sleeve 46.

The electrical circuit of the detector is completed by a DC source 48and current indicating means shown diagrammatically at 50.

The function and operation of the electron capture detector is wellknown. Briefly, in the absence of a sample in the body of detector 10 astanding current is established in the external electrical circuit dueto ionization of a non-electron-capturing carrier gas such as nitrogenintroduced through the electrode 26, by the ionising source 34. In thepresence of an electron-capturing sample material introduced into thedetector, the standing current reduces by an amount related to thenature and quantity of the sample introduced.

Sample flow into the detector 10 is effected through the aperture 32 andis achieved by application of a varying pressure to the internal volumeof the enclosure 14 by the electrical energisation of the loudspeaker 16causing the cone 21 to move in accordance with the energisation andthereby apply a pressure pulse to the inner volume of the enclosure 14which is communicated to the inner face of the plug 30 via the apertures40 in the detector body 24.

Application of a suitably polarised drive potential to the motor of theloudspeaker 16 to cause the cone 21 to move outwardly from the end face18 results in a negative pressure pulse being communicated to theinterior of the enclosure 14 and hence to the inner end of the pin-holeaperture 32, causing a sample of the atmosphere adjacent the exterior ofthe plug 30 to be drawn into the detector 10 and moved through theionising region of the detector under the influence of the carrier gaswhich is flowing from the mouth of the electrode 26 toward the apertures40. Removal or reversal of the drive potential on the motor of theloudspeaker will cause the cone 21 to move towards the end face 18 andresult in the expulsion from the interior of the enclosure 14 throughthe pin-hole aperture 32 of a similar volume of the enclosed atmosphereto that of the external atmosphere previously drawn in.

If a stream of sample gas from the exterior of the enclosure 14 isrequired to be introduced into the detector 10 this is achieved byapplication of a repetitive drive signal to the motor of the loudspeaker16 causing repetitive movement of the cone 21 and thus repetitivepulsing of the inner atmosphere of the enclosure 14. This will result inthe repetitive drawing in of samples of the atmosphere from the exteriorof the enclosure 14 which, by the appropriate choice of parameters forthe whole system, will enable a net transfer of sample gas from theexterior into the detector 10 as incoming samples will be swept from theregion of the plug 30 by the carrier gas flow following each inspirationand the atmosphere expelled through the pin-hole aperture 32 will belargely composed of carrier gas from the electrode 26.

The tube 42 is chosen with dimensions to offer a minimal resistance tothe outflow of carrier gas and sample mixture from the enclosure 14 but,due to the mass of gas contained within it, maximum impedance topressure pulses developed by the loudspeaker 16.

In the system described in relation to FIG. 1, the aperture 32 was 2 mmlong and 0.79 mm in diameter. The external dimensions of the enclosure14 were 80 mm long by 60 mm in diameter. The tube 42 was 50 mm long and3 mm in diameter.

Nitrogen carrier gas was introduced into the detector through theelectrode 26 at a flow-rate of 1.67 mls per second.

The loudspeaker 16 with a nominal cone diameter of 50 mm was driven witha sine-wave signal of 60 Hz from a variable frequency oscillator, andthe amplitude of the drive signal varied to vary the rate ofintroduction of air through the pin-hole aperture 32.

The relationship between the electron capture detector current and airflow through the aperture 32 was determined by a separate experiment inwhich a measured flow of air was applied to the aperture 32 and theresulting current noted. The air flow caused by the loudspeaker 16 wasthen deduced from the change in electron capture detector current. Hencethe plot shown in FIG. 2 was derived in which the induced air flowversus the peak-to-peak value of the drive signal at a constant 60 Hz isplotted. From this figure it will be seen that within the chosen drivesignal range, a near-linear relationship between drive signal andinduced airflow is demonstrated.

When freon gas of undetermined concentration was introduced into theregion of the exterior of the aperture 32 the reduction of detectorstanding current against the peak-to-peak drive signal at a constant 60Hz was as shown in the plot of FIG. 3.

Employment of a fluid sampling system in accordance with an embodimentof the invention, in an ion mobility spectrometer, offers a number ofadvantages over arrangements currently used which most commonly employ amembrane inlet system such as is described and illustrated in UK PatentNo. 2052750.

A schematic diagram of an ion mobility spectrometer employing a fluidsampling system in accordance with the invention is shown in FIG. 4. Thefunction and use of such instruments is well known in the art, forexample from UK Patent Application No. 2217103A, and will not be furtherdescribed here except to the extent necessary to illustrate theapplication of the present invention hereto.

Referring to FIG. 4, an ion mobility drift cell 60 has an inlet chamber62 adapted to receive a flow of gas or vapour 64 which is drawn throughthe chamber 62 to an outlet 66 by means of a fan or pump 68 andexhausted at a vent 70. Preferably a fan 68 is used rather than a pumpto avoid undesirable pressure oscillations arising with the system.

The drift cell 60 is separated from the inlet chamber 62 by a wall 72 inwhich there is a pin-hole aperture 74.

The drift cell 60 is connected into a closed loop circulatory carriergas system comprising return flow line 76, a recirculatory fan or pump78, a transfer line 80, a sieve pack 82, a transfer line 84, a manifold86, a source flow line 88 and a drift flow line 90. Preferably a fan 78is used rather than a pump, to avoid undesirable pressure oscillationsarising within the system. The circulatory carrier gas is air.

An electromechanical transducer 92 is pneumatically coupled to the line80 and between the fan 78 and the sieve pack 82 through a line 94 and isdriven from a source of alternating current 96 connected to it by aswitch 98 to produce repetitive pressure pulsing of the line 80 and thusof the whole closed loop circulatory system when switch the 98 isclosed. The transducer could be located elsewhere within the sealedcirculatory system.

In operation, with the switch 98 open, a flow of external atmosphere 64is drawn through the inlet chamber 62 into the line 66, through the fan68 and is returned to atmosphere through the vent 70. Only a little ofthe inlet flow enters the drift cell 60 through the pin-hole aperture74, as the dimensions of the aperture 74 constitute a large diffusionbarrier to entry. Alternatively (not shown), gas could be injected intothe chamber 62 from a high-pressure source and vented to atmospherewithout the use of the fan 68.

The aperture 74 is 0.9 mm in depth and 0.3 mm in diameter. The exactdimensions are not critical but of course the smaller and deeper thehole the greater the resistance against diffusion from the inlet chamber62, and the smaller the sample drawn in at each pressure pulse. Theaperture should not be so large that bulk flow of gas is possiblethrough it, except when a pressure pulse is applied. Larger apertures ofcourse need smaller pressure pulses for a given sample size.

A circulatory flow of carrier gas, for example dry air, is maintained inthe sealed circulatory loop by the fan 78. A primary flow passes intothe drift cell 60 from the source flow line 88 into the region of thewall 72 passing through the reaction chamber part of the cell andexhausting to the return flow line 76. A secondary flow passes into thecollector region of the drift cell 60 and passes down the length of thedrift cell 60 also to exhaust to the return flow line 76.

With the switch 98 closed repetitive pressure pulses (e.g. at a few tensof hertz) are applied to the circulatory loop, and via the loop to theregion in the drift cell 60 adjacent the wall 72 and the pin-holeaperture 74. Successive negative-going pulses will cause successivesamples of the inlet flow 64 to be drawn from the chamber 62 through theaperture 74 into drift cell where they are entrained in the source flowand swept through the reaction chamber of the cell 60 to the return flowline 76. Positive-going pressure pulses will eject a discrete amount ofcarrier gas from the cell 60 through the aperture 74 but little or noneof the previously incoming sample of inlet flow, resulting in a netinflow of samples from the external atmosphere into the cell 60 fordetection and or measurement.

It will be appreciated that the magnitude of the sample flow enteringinto the drift tube 60 will be controllable by control of the drivesignal applied to the transducer 92 from the supply 96.

It will also be appreciated that the drive signal applied to thetransducer 92 could by means of a suitable feedback loop be varied independence upon the magnitude of the electrical output signal derivedfrom the drift tube 60 such as to increase or decrease the amount ofsample incoming to the tube 60, thereby controlling the sensitivity ordynamic range of the instrument in a manner well known per se in theart. Also, if the drive signal is removed, the behaviour of the IMS cell60 in the absence of sample flow through the aperture 74 can bemonitored.

FIG. 5 shows schematically a suitable electromechanical transducer foruse in the ion mobility spectrometer described with reference to FIG. 4.The transducer is a modified loudspeaker comprising a motor 100, and aframe 102 supporting a moving cone (not shown) attached to a rim 104.The airspace forward of the cone is sealed by means of a plate 106 gluedto the rim 104 by glue 108 to give a hermetically sealed enclosure, theonly communication to which is through a pipe 110 mounted into asuitable aperture in the plate 106. The pipe 110 is coupled to the line94 of FIG. 4 which in turn is connected to the closed loop circulatorysystem of the ion mobility spectrometer.

Although the invention has been described with specific reference toprovision of pressure pulsing by use of an electromechanical acoustictransducer, it will be apparent that other means of providing therequisite pressure pulsing may be employed, for example, apiezo-electric actuator where an applied electrical signal produces amechanical deformation of the crystal which could be applied to aflexible conduit to produce the necessary pressure pulsing.

Embodiments of the invention could be used as continuously orrepeatingly operated detectors in a process line, or could be used forcontinuous or user-controlled detection of small quantities of noxiousor other gases in the ambient atmosphere.

The invention may also be used other than in the context of analyticalinstrumentation, for example for introducing a controlled quantity of agas or vapour of known concentration into an enclosed static or flowingbody of gas or vapour, or to permit injection of a controlled amount ofa gas, vapour or liquid into another medium, for example in a chemicalprocess plant. Embodiments of the invention could be used to provideliquid samples into a static or flowing body of liquid. They could alsoprovide liquid samples into a static or flowing body of air or othergas.

We claim:
 1. A fluid sampling system for extracting a fluid sample from a body of fluid, the system comprising fluid enclosing means arranged to enclose a volume of a first fluid into which a sample of a second fluid is to be introduced, the fluid enclosing means comprising a substantially closed chamber having an orifice via which the second fluid may be drawn into the fluid enclosing means, and means for drawing the sample of the second fluid into the volume of first fluid through the orifice, comprising a loudspeaker including a diaphragm arranged to apply a negative pressure pulse to the first fluid, and in which the negative pressure pulse applied to the first fluid is applied directly to the second fluid via the orifice, whereby a differential pressure is caused to exist across the orifice.
 2. A fluid sampling, system as recited in claim 1, including means for selectively adjusting the loudspeaker to vary the amplitude of the pressure pulse.
 3. A fluid sampling system as recited in claim 1, including means for selectively adjusting the loudspeaker to vary the duration of the pressure pulse.
 4. A fluid sampling system as recited in claim 1, including driving means arranged to drive the loudspeaker repetitively, so effecting repeated transfers of samples of the second fluid into the fluid enclosing means.
 5. A fluid sampling system as recited in claim 1, including means for adjusting the driving means so as selectively to vary a repetition rate of the loudspeaker. 